It is apparent that, in future, new battery systems will be increasingly used both in stationary applications, such as wind turbines, in vehicles, such as in hybrid and electric vehicles and in the consumer sector, such as in laptops and mobile telephones, with very stringent requirements being placed on said battery systems in respect of the reliability, safety, performance and service life thereof.
Batteries using lithium-ion technology are particularly suitable for such tasks. They are distinguished, inter alia, by high energy density and a low self-discharge. By definition, lithium-ion batteries consist of two or more lithium-ion cells which are mutually interconnected. Lithium-ion cells can be connected in parallel or in series to form modules and then interconnected to form batteries. In this case, a battery module usually consists of six or more cells.
In order to ensure reliable and correct functioning of a sufficiently long service life, lithium-ion cells may be connected to a battery management system, by means of which the battery is monitored and regulated. In this case, the battery management system performs a multiplicity of tasks, such as balancing, temperature regulating, etc. at the battery cells, which are necessary for the operation of the battery cells. In particular, the electrical voltage of the individual battery cells is also monitored, particularly so that no overloading or insufficient charging takes place.
The basic circuit diagram of a battery management system 100 of a traction battery is illustrated in FIG. 1. The architecture shown there consists of a so-called battery control unit 101 (BCU) and at least one or more so-called cell monitoring units 102 (“cell supervision circuits”) (CSC) which are connected in situ via a communication bus 114 to the battery modules (not labeled). In this case, a cell monitoring unit 102 monitors the battery cells 103 of one or two battery modules, for example. The cell monitoring units 102 are present in a sufficient number to monitor the large number of—for example up to a hundred—battery cells 103 of a traction battery. Depending on which demands are placed on the performance of the battery system, the BCU and CSC electronics can be arranged on a common printed circuit board. As also shown in FIG. 1, the total voltage of the battery is tapped via a positive battery voltage supply line 104 and a negative battery voltage supply line 105 by the battery control unit in order to then be further processed in the battery control unit. Furthermore, the battery control unit 101, in addition to a charging contactor 106, also actuates a positive battery contactor 107 and a negative battery contactor 108, by means of which the positive battery cell terminal 109 and the negative battery cell terminal 110 can in each case be switched to be voltage-free. In particular, by actuation and subsequent opening of the contactors, a disconnection of the battery can be achieved in the event of an overcurrent or a short circuit, as a result of which a dangerous situation caused by the high battery voltage can be avoided in the event of an emergency. Current sensors 111, 112 are used to determine the battery current. Furthermore, the battery control unit 101 can be connected, for example by means of a CAN bus 113, to a central vehicle controller (not shown).
FIG. 2 is a basic illustration of an exemplary input circuit of a cell monitoring unit 200. As shown in FIG. 2, the cell monitoring unit 200 is constructed in a modular fashion and has a monitoring component denoted as companion chip 201 which controls the adherence to the permissible cell voltage range, and a so-called front-end chip or main chip 202 with which the cell voltages are measured. The front-end chip 202 comprises an analog-to-digital converter 203 and a control and communication unit 204 which can output and read data via daisy-chain connections 205, for example. Furthermore, the input circuit of the cell monitoring unit 200 has a filter 206 and a resistively operating balancing circuit 207. The companion chip 201 can be designed as a threshold voltage comparator and, when respective voltage limits are exceeded or undershot, it can output an alarm 208 and activate a hardware release cord, for example in order to open the contactors shown in FIG. 1. As shown in FIG. 2, the voltage limits in the case of lithium-ion cells can be 2.6 V or 4.2 V. The cell voltage is determined as an input voltage at the cell voltage connections 209.
A cell monitoring unit which has a companion chip is described, for example, in an earlier patent application by the applicant, with the application number DE 10 2011 079 120 A1.
What is disadvantageous in the known battery management systems is that monitoring which also includes negative input voltages, in particular when several battery cells to be monitored are present, would require additional high expenditure in terms of circuitry which would be connected to high costs. For this reason, the known battery management systems are often not configured to be sufficiently robust with respect to negative input voltages.